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As many physical properties of polymers scale with molecular weight, the ability to achieve polymers of nearly inaccessibly high molecular weight provides an opportunity to probe the upper size limit of macromolecular phenomena. Yet many of the most stimulating macromolecular designs remain out of reach of current ultrahigh molecular weight (UHMW) polymer synthetic approaches. Herein, we show that UHMW polymers of diverse composition can be achieved by irradiation of thiocarbonylthio photoiniferters with long-wave ultraviolet or visible light in concentrated organic solution. This facile photopolymerization strategy is general to acrylic-, acrylamido-, methacrylic-, and styrenic-based monomers, enabling the synthesis of well-defined macromolecules with molecular weights in excess of 10⁶ g/mol. The high chain-end fidelity afforded by photoiniferter polymerization conditions facilitated the design of UHMW …